Hybrid vehicles, such as plug-in hybrid vehicles, may have two modes of operation: an engine-off mode and an engine-on mode. While in the engine-off mode, power to operate the vehicle may be supplied by stored electrical energy. While in the engine-on mode, the vehicle may operate using engine power. By switching between electrical and engine power sources, engine operation times may be reduced, thereby reducing overall carbon emissions from the vehicle. However, shorter engine operation times may lead to insufficient purging of fuel vapors from the vehicle's emission control system. Additionally, refueling and emission control system leak detection operations that are dependent on pressures and vacuums generated during engine operation may also be affected by the shorter engine operation times in hybrid vehicles.
Various strategies have been developed to address fuel vapor control and management in hybrid vehicle systems. Example approaches include separating storage of refueling vapors from storage of diurnal vapors by adding a fuel tank isolation valve (FTIV) between a fuel tank and a fuel vapor retaining canister, and allowing refueling vapor purging to the canister during refueling events, and engine-on purging methods. The separation of diurnal and refueling vapors allows a pressure to be generated in the fuel tank, while application of alternative vacuum sources allows a vacuum to be generated in the canister.
One example approach for fuel vapor management is shown by Suzuki et al. in U.S. Pat. No. 7,032,580 B2. Therein, a control system determines if the fuel tank pressure is above a predetermined value while a purging from the carbon canister to the engine intake is occurring. If the fuel tank pressure is greater than the predetermined value, a pressure control valve between the fuel tank and the carbon canister is opened to permit purging of fuel vapors from the fuel tank. Opening of an exhaust gas recirculation valve is adjusted based on the opening of the pressure control valve to decrease an amount of fuel injected to the engine and suppress fluctuation of the air-fuel ratio. The pressure control valve is closed after a duration, based on pressure in the fuel tank and the purge flow rate, has elapsed.
However, the inventors herein have recognized potential issues with this approach. As one example, under conditions where the average fuel tank pressure is very high, such as in hybrid vehicle fuel systems, the pressure valve may open more frequently, or for longer durations, leading to more frequent engine air-fuel ratio fluctuations. For example, if the fuel tank is normally sealed from the canister, the ambient temperature is high, and the hybrid vehicle has been operated in the engine-off mode for a long duration, the fuel tank pressure may be significantly higher than that of the carbon canister. As a result, controlled release of fuel vapors from the fuel tank, to dissipate the fuel tank pressure, while the canister purge valve is open may be difficult to achieve, and flooding of the engine with excessive vapors may occur. Such flooding may lead to engine stumbles and stalls, and degrade engine performance.
The above issue may be at least partly addressed by a method that allows for coordination of purging fuel vapors from the fuel tank with purging of fuel vapors from the canister, by regulating opening of a FTIV, to maintain a canister pressure and a fuel tank pressure, without flooding the engine. In one embodiment, the method includes, operating a fuel vapor recovery system coupled to an engine intake, the fuel vapor recovery system including a fuel tank coupled to a canister through a first valve, the canister coupled to the engine intake through a second valve, the canister further coupled to a vacuum accumulator. The method may comprise, during a purging condition, purging a first amount of fuel vapors from the canister to the engine intake; and after purging the first amount, purging a second amount of fuel vapors from the fuel tank to the canister, the second amount adjusted based on the first amount.
In one example, a fuel vapor recovery system for a hybrid vehicle may comprise a fuel tank coupled to a canister via a fuel tank isolation valve (FTIV). The canister may be coupled to the engine intake via a canister purge valve (CPV). The canister may be further coupled to a vacuum accumulator via a vacuum accumulator valve (VAV). As such, the FTIV may be maintained closed during vehicle operation and may be selectively opened during refueling and diurnal purging conditions. By maintaining the FTIV closed, the fuel vapor circuit may be divided into a canister side and a fuel tank side. Refueling vapors may be retained in the canister on the canister side of the circuit while diurnal vapors may be retained in the fuel tank on the fuel tank side of the circuit.
A first pressure sensor may be coupled to the fuel tank to estimate a pressure of the fuel tank side of the circuit, while a second pressure sensor may be coupled to the carbon canister to estimate a pressure of the canister side of the circuit. Based on input from various sensors, such as the pressure sensors, and further based on vehicle operating conditions, a controller may adjust various actuators, such as the CPV, the FTIV, and a canister vent valve (CVV), to enable fuel tank refueling, purging of stored fuel vapors, and leak detection in the fuel vapor recovery system.
In one example, the controller may determine purging conditions are met when the canister pressure is above a first predetermined threshold while the vehicle is operated in an engine-on mode. The controller may then open the CPV to purge refueling vapors stored in the carbon canister to the engine intake. Once the carbon canister pressure has fallen below the first threshold, the CPV may be closed and the controller may determine if a fuel tank pressure is above a second predetermined threshold. The controller may accordingly open the FTIV to purge diurnal vapors stored in the fuel tank to the carbon canister. The duration of opening of the FTIV may be determined as a function of the canister pressure, so that the controller may close the FTIV as the pressure of the canister approaches the first threshold.
In such an approach, operation of the FTIV may be tightly regulated during purging to maintain a pressure in the carbon canister that is below the first threshold for purging and prevent flooding of the engine during a purging operation. Additionally, regulated bleeding of fuel vapors from the fuel tank may partially curb the high pressure fluctuations experienced in the fuel tank. Further, the present approach may allow fuel tank and canister pressures to equilibrate faster. As such, fuel tank refueling may be unsafe when fuel tank pressures are high. Thus, by equilibrating fuel tank and canister pressures faster, purging may be better coordinated with refueling operations, for example, by reducing a delay time between receiving a refueling request event and opening of a fuel tank door, after fuel tank pressure dissipation, to receive fuel.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.